The present application claims priority under 35 U.S.C. xc2xa7119 of German Patent Application No. 101 35 957.8, filed Jul. 24, 2001, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The present invention relates to a magnetic recording medium comprising a nonmagnetic substrate and a lower layer which is applied thereon and contains at least one inorganic pigment and at least one binder, and a magnetic layer which is applied on the layer and contains a magnetic pigment, at least one binder and a lubricant and at least one nonmagnetic pigment.
2. Discussion of Background Information
Modern magnetic recording media will meet the steadily growing requirements only when they permit larger storage capacities, shorter access time and higher transmission rates of the stored information. These recording media must have a composition such that magnetic pigments have a sufficiently high density in the recording layer in order to ensure high storage capacities, but the magnetic layer thickness must be very small in order to permit direct overwriting of the data on rerecording without a prior erasing process. For example, magnetic recording media having a high storage capacity today have magnetic layer thicknesses of only less than about 1 xcexcm.
There are substantially two different methods for producing a magnetic recording medium for a high recording density. According to the first method, a magnetic metal or a magnetic metal alloy is applied to a substrate under greatly reduced pressure in order to thereby produce a magnetic metal film having a very small thickness. Such recording media have good recording properties, in particular a very high recording density, but can be produced only in a small amount since the application of the magnetic layer under reduced pressure constitutes a very complicated process. Moreover, since the magnetic metal pigment is present in unprotected form, such recording media tend to oxidize very easily, which has a very adverse effect on the long-term stability.
There are magnetic recording media in which the magnetic metal pigment is embedded in a binder matrix and is applied to a nonmagnetic substrate in a coating apparatus. In order to achieve a high sensitivity here too, the thickness of the magnetic layer must be kept very small. However, this gives rise to problems owing to the roughness of the magnetic layer, and consequently leads to poor stability of such magnetic layers. In order to overcome this obvious disadvantage, a nonmagnetic layer is initially applied to the nonmagnetic substrate and it is only on the nonmagnetic layer that a very thin magnetic layer is applied. The nonmagnetic layer acts as a buffer layer into which the upper thin magnetic layer can partly penetrate, it being possible for problems due to surface roughness to be substantially avoided. Such magnetic recording media are described, for example, in EP-A-0 520 155, which is incorporated by reference herein in its entirety.
A recording medium of the type stated at the outset is disclosed in U.S. Pat. No. 6,037,037, which is incorporated by reference herein in its entirety. It has, in the lower layer, an acicular pigment having a needle length of from 0.25 to 1 xcexcm. In the upper magnetic layer, a binder having a low Tg of 30xc2x0 C. and one having a higher Tg of 70xc2x0 C. are used. Such a medium has too high a surface roughness for high-density recordings, owing to the coarse-particled pigment in the lower layer, and an insufficient freeze-frame time because of the admixing of a relatively soft binder in the upper layer.
A recording medium which permits a high recording density is also required to have the following properties in addition to good electromagnetic properties:
A. The frictional contact between head and tape surface must be kept to a minimum, something which most manufacturers of magnetic recording media attempt to achieve by adding lubricants to the upper and the lower layer.
B. The magnetic recording medium may cause only slight wear of the heads, which is limited by a specification. This requirement can be met, for example, if the upper layer contains a very soft binder in which hard nonmagnetic pigments are incorporated.
C. Even during prolonged operation, the magnetic recording medium should exhibit no significant abrasion, which can cause the head surface to stick. This requirement also applies in the case of extreme climatic conditions, for example in a very humid or very dry climate or at relatively high temperatures. The last-mentioned requirement is met if the upper layer has a very hard surface, which however entails the risk of embrittlement of the layer.
From the above, it is evident that, in particular, the requirements B and C may contradict one another. In order to comply with both requirements, neither abrasion nor wear of the head can be optimally established, with the result that the quality of the magnetic recording medium is adversely affected.
The present invention relates to magnetic recording medium of the type stated at the outset, in which the friction between tape surface and head is greatly reduced and which at the same time causes only slight wear of the head. In particular, only very slight or no deposits should occur on the head in a dry climate and at relatively high temperatures. Moreover, such a magnetic recording medium should have a very thin upper magnetic layer in order to have good recording and playback properties. Moreover, the recording medium should be capable of being produced simply and in large quantities.
The present invention provides a magnetic recording medium comprising
a) a substrate,
b) at least one lower layer which is applied thereon, and contains at least one binder and at least one inorganic pigment, and
c) at least one upper magnetic layer which is applied on the lower layer and contains at least one magnetic pigment P2 and at least one binder and at least one further, nonmagnetic pigment P3,
(1) the binder matrix of the upper magnetic layer having a Tg of at least 50xc2x0 C. and the binder matrix of the lower layer containing at least one binder having a Tg less than 50xc2x0 C. and hence determining the viscoelastic properties of the composite comprising magnetic and nonmagnetic layers;
(2) the further nonmagnetic pigment P3 of the upper magnetic layer having a particle size which corresponds at least to the dry thickness of the upper magnetic layer; and
(3) the upper magnetic layer containing at least one compound (A) and at least one compound (B), (A) corresponding to the following formula
R1xe2x80x94COOxe2x80x94(R3xe2x80x94O)mxe2x80x94R2xe2x80x83xe2x80x83(A)
xe2x80x83where
R1 is a saturated or mono-or polyunsaturated, straight-chain or branched hydrocarbon radical of 11 to 22 carbon atoms;
R2 is a saturated or mono-or polyunsaturated, straight-chain or branched hydrocarbon radical of 1 to 6 carbon atoms;
R3 is a straight-chain or branched alkylene radical of 1 to 4 carbon atoms; and
m is an integer from 1 to 4;
and (B) corresponding to the following formula:
R1xe2x80x94COOxe2x80x94R4xe2x80x83xe2x80x83(B)
xe2x80x83where
R1 is as defined above and
R4 is a saturated or mono-or polyunsaturated, straight-chain or branched hyrocarbon radical of 4 to 8 carbon atoms.
In other words, the present invention is directed to a magnetic recording medium comprising:
a) a substrate,
b) at least one lower layer on said substrate, said at least one lower layer containing at least one binder in a binder matrix and at least one inorganic pigment, and
c) at least one upper magnetic layer on said at least one lower layer, said at least one upper layer containing at least one magnetic pigment P2 and at least one binder in a binder matrix and at least one nonmagnetic pigment P3,
(1) the binder matrix of the at least one upper magnetic layer having a Tg of at least 50xc2x0 C. and the binder matrix of the at least one lower layer containing at least one binder having a Tg less than 50xc2x0 C.;
(2) the at least one nonmagnetic pigment P3 of the at least one upper magnetic layer having a particle size which corresponds at least to the dry thickness of the upper magnetic layer; and
(3) the upper magnetic layer containing at least one compound (A) and at least one compound (B).
The at least one lower layer can contain at least one compound (A) and at least one compound (B).
The binder matrix of the at least one upper layer can contain at least one binder having a Tg of at least 50xc2x0 C., which comprises at least one of vinyl copolymers, phenol resins, acrylates, epoxy resins, cellulose resins, polyesters and polyurethanes.
The binder matrix of the at least one upper layer can contain at least one binder having a Tg of at least 50xc2x0 C., which comprises at least one of vinyl copolymers, phenol resins, acrylates, epoxy resins, cellulose resins, polyesters and polyurethanes.
The binder matrix of the at least one lower layer can contain at least one binder having a Tg of less than 50xc2x0 C., which comprises at least one of polyurethanes, rubbers, vinyl polymers, phenol resins, acrylates, epoxy resins, cellulose resins, and polyesters.
The at least one binder of the at least one upper layer and/or the at least one lower layer can contain polar groups.
The binder matrix of the at least one lower layer can further comprise at least one binder having a Tg of at least 50xc2x0 C., which comprises at least one of vinyl copolymers, phenol resins, acrylates, epoxy resins, cellulose resins, polyesters and polyurethanes.
The compounds (A) and (B) in the at least one upper layer can be contained in an amount in the range from about 0.2 to 10% by weight, based on the magnetic pigment.
The weight ratio of compound (A) to compound (B) in the at least one upper layer can be in the range from 20:80 to 80:20.
The at least one lower layer can contain at least one pigment having a BET value of at least 40 m2/g.
The at least one upper layer can further comprise a nonmagnetic pigment P4 having a particle size which is smaller than the dry thickness of the at least one lower layer.
The nonmagnetic pigments P3 and P4 of the at least one upper layer can comprise of at least one of metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides and metal sulfides.
At least one of the pigment P3 and the pigment P4 can be an Al2O3.
The pigment P3 having a larger particle size and the pigment P4 having a smaller particle size can be contained in a weight ratio of pigment P3 to pigment P4 of from 80:20 to 20:80.
The at least one inorganic pigment of the at least one lower layer can comprise at least one of metal oxides and carbon blacks.
The at least one magnetic pigment P2 in the at least one upper layer can be a metal powder or an alloy powder having a coercive force of at least 100 kA/m.
The at least one magnetic pigment P2 of the upper layer can be a highly coercive ferrite. The ferrite can be barium ferrite.
The at least one inorganic pigment in the lower layer can comprise at least one of at least one nonmagnetic pigment P1 and at least one magnetically soft pigment P5 having a coercive force of less than 0.7 kA/m.
The compound (A) can comprise butoxyethoxyethyl stearate. The compound (B) can comprise isobutyl stearate.
The invention is also directed to processes for production of a magnetic recording medium, such as wherein the at least one upper layer is applied to an already dry at least one lower layer present on the substrate; or the at least one upper layer is applied to a wet at least one lower layer present on the substrate, or wherein both layers are applied simultaneously to the substrate. The medium can be dried after application of the at least one upper magnetic layer and is then subjected to a calendering process with the use of pressure, and optionally an elevated temperature.